1. Technical Field
The present application is directed to a method for leveling a substrate and an article formed thereby and, in particular, to a method for providing a level surface onto which a coating may be applied.
2. Related Art
Physical Vapor Deposition (PVD) processes are capable of providing relatively thin (typically in the micron range) metallic coatings that have high luster and superior wear characteristics. These characteristics are desirable in many applications and, in particular, for decorative coatings. However, the characteristics may be affected by the quality of the substrate. Generally, because PVD coatings are thin, they cannot eliminate surface defects such as pores or surface roughness, and in many instances surface defects are highlighted. For example, a PVD coating applied to a rough or brushed substrate will be dull, because the PVD coating conforms to the surface on which it is applied. In addition to affecting the aesthetic characteristics of PVD coatings, porous or rough substrates may have substances trapped in pores or depressions. When such substances outgas before or during the PVD process, it may be detrimental to the PVD equipment or coating, even when present in minimal amounts. For example, outgassing substances may prevent the PVD equipment from achieving the proper vacuum level, or may evaporate and be co-deposited with the PVD coating material.
For the foregoing reasons, PVD coatings have generally been limited to substrates having smooth or non-porous surfaces, or surfaces that can be made smooth or non-porous. Thus, PVD coatings have not been used in many otherwise suitable applications. For example, certain porous substrates such zinc, brass, steel, aluminum and tin are desirable in many applications because they are relatively inexpensive. Methods have therefore been devised to “level” the surface of rough or porous substrates. Such “leveling” methods in some instances seal the substrate surface, providing improved corrosion resistance and promoting the adhesion of coatings to the substrate surface.
One method that has been used to level porous or rough surfaces is by polishing. PVD coated polished surfaces generally will be shiny, whereas an unpolished or brushed part will provide a dull coating. Surfaces may be polished using mechanical abrasion, chemical etching, or a combination of both.
Another method that has been used to level porous or rough substrates is to plate the surface, generally with copper, nickel, and chrome, which also improves corrosion resistance and provides a surface to which coatings adhere well. During the plating operation, surface defects are filled in by the plated material, thus leveling the surface. However, many plating operations, in particular chrome plating, are expensive and may be harmful to the environment. In some cases, plating results in an undesirable galvanic coupling.
In addition, substrate preparation processes are time consuming, expensive, and labor intensive. Generally, only premium applications can justify either the high cost of base materials such as stainless steel, or the costs of pre-finishing less expensive substrates before metallizing.
Thus, there is need in the art for alternative methods for substrate preparation and coating substrates.